Ch 14: Solutions

1. Ch 14: Solutions Solutions are homogeneous mixtures (solute + solvent). Solute is the dissolved substance. Seems to “disappear” in the solvent. Solvent is the substance in which the solute dissolves. Does not appear to change state. Aqueous solutions Have water as the solvent. Water is called the universal solvent because it dissolves so many substances. Metal solutions (one metal dissolved in another) are called alloys (ex: steel, brass, bronze). See table 14.1 for all types of solutions

2. 14.1 Solubility When one substance (the solute) dissolves in another (the solvent) it is said to be soluble. Ex: salt and ethanol are both soluble in water When one substance does not dissolve in another it is said to be insoluble. Ex: oil is insoluble in water When a solute and solvent have the same state (gas, liquid, or solid), the solvent is the component present in the highest percentage. Metal solutions (one metal dissolved in another) are called alloys. Ionic Compounds Dissociate into ions when dissolved in water Polar water molecules interact with positive and negative ions to dissolve the compound (fig 14.1, 14.2) ions surrounded by water molecules are hydrated

3. Covalent Compounds Polar covalent compounds have “polar” groups that tend to be soluble in water. Ethanol (a liquid) or sucrose (table sugar), for example, have polar O-H bonds that interact with water to enhance solubility. See fig 14.3, 14.4 Nonpolar covalent compounds have molecules that do not form attractions to water molecules because all their bonds are essentially non-polar, which prevents them from being soluble in water. Petroleum products or vegetable oil molecules are examples. See figs 14.5, 14.6

4. Molecules that are similar in structure tend to form solutions: “like dissolves like” A given solvent dissolves solutes that have polarities similar to its own. Water (a polar molecule) dissolves most polar solutes (liquids or solids). Non-polar solvents dissolve nonpolar solutes. Gasoline, for example, dissolves in oil since they are both nonpolar compounds. However, neither gasoline nor oil will dissolve in water because water is polar. Gases are always soluble in each other.

5. 14.2 Solution Composition Concentration = amount of solute in a given amount of solution. A concentrated solution has a high proportion of solute to solution. A dilute solution has a low proportion of solute to solution. A saturated solution contains the maximum amount of solute that will dissolve in the solvent at a particular temperature. (The higher the temperature the greater the saturation.) An unsaturated solution contains less solute than the saturation limit. A supersaturated solution contains more solute than the saturation limit. Unstable; the solute easily precipitates out of solution.

6. 14.3 Solution Composition: Mass Percent Mass percent is an expression of the mass of solute present in a given mass of solution. Mass Percent = (grams of solute ÷ mass of solution) x 100 Remember, the mass of the solution is the sum of the masses of the solute and the solvent. See examples 14.1, 14.2

7. 14.4 Solution Composition: Molarity Describes a solution in terms of concentration, or the amount of solute in a given volume. Molarity (M) = moles of solute per 1 liter of solution. A standard solution is a solution whose concentration is accurately known. See Fig 14.7 for preparation. Molarity calculations: See examples 14.3, 14,4, 14.6, 14.7 in text Calculate molarity Calculate moles/mass of solute Calculate volume of solution

8. 14.5 Dilution Dilution is adding solvent to decrease the concentration of a solution. See Fig 14.8 and example 14.8. The amount of solute stays the same, but the solute concentration decreases due to the added solvent. Dilution Formula: M1 x V1 = M2x V2 M1V1 = before dilution; M2V2 = after dilution Since molarity is defined in terms of liters, change any volume in milliliters to liters to solve dilution problems. Examples: